An apparatus of the kind described above is disclosed in U.S. Pat. No. 4,123,227 incorporated herein by reference. In the gas measuring and warning apparatus disclosed in this patent, a sample quantity of the gas to be detected is passed through the testing tube by means of an air-pumping device. The presence of a specific pollutant to be detected leads to a coloration reaction with the indicator contained in the testing tube. After the sample is taken, the coloration zone is scanned with the aid of a row of mutually adjacent diodes and receivers which define respective light barriers. The length of the coloration zone detected in this manner is applied to an evaluation unit by means of which corresponding measurement values are determined and displayed.
The detection method using colorimetric testing tubes can however not be carried out for all types of pollutants in the same manner. Depending upon the pollutant to be detected, other charges of the testing tube, other indicators, and therewith different sample quantities must be pumped through the testing tube. In addition to these characteristics, not only is the sample quantity to be pumped in accordance with the desired detection sensitivity different, but also the length of the coloration zone is different. In this way, the length of the coloration zone when detecting the one gas corresponds to a specific concentration which, when detecting another gas, can lead to a distinctly different length of the corresponding coloration zone. The individual data characteristic for the testing tube are therefore not available to the evaluation unit so that the display is correct only for a predetermined type of testing tube.